An inductor alternator flywheel system, for converting between electrical and kinetic energy, includes an annular steel flywheel mounted on bearings for rotation about an axis. The flywheel has an inwardly facing radial surface forming multiple protrusions or teeth extending radially inwardly. A yoke has a field coil for producing homopolar flux which creates magnetic poles in the teeth. A cylinder constructed of substantially high permeability material, and having an outer radial surface, is mounted concentrically in the bore of the annular flywheel and is spaced apart radially from the teeth such that an armature air gap is formed between the teeth and the outer surface of the cylinder. A ring of armature coils is mounted within the air gap such that the flux induces an alternating voltage in the armature coils when the rotor rotates about its axis. A field controller provides field power at a level to the field coil when the flywheel is rotating at normal fill operating speed such that the generator provides full output power under full load to the output at the instant of disruption in primary power. The field controller increases the field power during a continuous disruption of primary power to maintain a substantially constant output voltage at the output as the generator speed decreases.
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38. An inductor alternator flywheel system for converting between electrical and kinetic energy, said apparatus comprising:
an annular steel flywheel rotor with a hollow center bore and an attached internal liner inside said bore having inwardly extending radial protrusions and being constructed from multiple axial layers that reduce eddy current generation in said protrusions; a source of homopolar flux that generates homopolar flux in a magnetic circuit that includes said flywheel rotor and at least one stationary armature coil mounted such that said flux induces an AC voltage in said armature coil when said rotor is rotated about an axis.
23. A apparatus for converting between electrical and kinetic energy comprising:
a flywheel rotor comprising a hollow cylinder and a liner with intrusions extending therefrom and forming a radial air gap, said intrusions being constructed of ferromagnetic material; a member that generates homopolar flux, said intrusions and said flux generating member forming a magnetic circuit having a total reluctance that remains substantially constant while said rotor is rotated about an axis; and at least one stationary airgap armature coil mounted within said radial airgap with the active portion of said armature coil extends to substantially the entire axial length of said intrusions such that flux induces an AC voltage in said at least one armature coil when said rotor is rotated about said axis.
30. An inductor alternator flywheel system comprising:
a rotatable member mounted for rotation about an axis, said member including a steel flywheel rotor with a hollow portion having a laminated liner with a plurality of intrusions extending therefrom to a minimum radius; a non-rotating member that generates flux, said flux creating magnetic poles in said intrusions; at least one cylinder having an outer surface comprised of laminations, said cylinder being mounted concentric with respect to said rotor such that an armature air gap is formed between said intrusions and said outer surface, said cylinder being constructed of substantially high permeability material; and at least one armature coil mounted within said air gap such that said flux induces an AC voltage in said at least one armature coil when said rotated about said axis.
31. An inductor alternator for a flywheel energy storage and retrieval system comprising:
a rotatable member that may be rotated about an axis, said rotatable member including a steel flywheel rotor with a hollow portion having a plurality of intrusions extending radially inwardly therefrom to a minimum radius; a non-rotating member that generates homopolar flux, said flux creating magnetic poles in said intrusions; a cylinder having an outer surface, said cylinder being mounted such that it rotates with said rotor such that an armature air gap is formed between said intrusions and said outer surface, said cylinder being constructed of substantially high permeability material; and at least one armature coil mounted within said air gap such that said flux induces an AC voltage in said at least one armature coil when said rotated about said axis.
19. An apparatus for providing emergency power in the event of disruption in power from a primary power source coupled to an output, said apparatus comprising:
a rotatable mass that stores kinetic energy; a brushless generator having a pair output leads, said leads being coupled to said output, and at least one field coil, said generator being coupled to said rotatable mass such that kinetic energy from said mass is converted to electrical energy and delivered to said leads when said field coil is energized; said field coil being sufficiently energized when said primary power source is operational such that loss of said power does not require a rapid increase in power to said field coil to prevent a period of disruption in output power; and a monitoring system that monitors the rotational speed and automatically increases the drive signal to said field coil to maintain a substantially constant output voltage at said output as said generator speed decreases.
27. An inductor alternator flywheel system comprising:
a rotatable member that may be rotated about an axis, said member including a steel flywheel rotor having a plurality of poles; a non-rotating member that generates homopolar flux, said flux magnetizing said poles; a stator having an air gap facing surface, said stator being mounted concentric with respect to said rotor such that an armature air gap is formed between said poles and said air gap facing surface, said stator being constructed of substantially high permeability material; and at least one armature coil mounted within said air gap such that said flux induces an AC voltage in said at least one armature coil when said rotated about said axis; said flywheel rotor being supported for rotation about a substantially vertical axis by a full levitation magnetic bearing system comprising at least one axial magnetic bearing carrying the axial weight and at least two radial magnetic bearings providing radial support.
34. An inductor alternator flywheel system comprising:
a rotatable member supported for rotation about a substantially vertical axis, said rotatable member including a flywheel rotor with a non-uniform surface such that variations in said surface form poles; a non-rotating member having at least one field coil that generates homopolar flux, said flux traveling through said poles; a stationary portion of said non-rotating member adjacent to and spaced apart from said poles for receiving said flux and defining an air gap between said stationary member and said poles; at least one armature coil mounted within said air gap such that said flux induces an AC voltage in said armature coil when said rotor is rotated about said axis; a full levitation magnetic bearing system for supporting said rotor for rotation about said vertical axis, said magnetic bearing system comprising an axial force generating magnetic bearing and upper and lower radial force generating magnetic bearings.
36. An apparatus for providing emergency power in the event of disruption in power from a primary power source coupled to an output, said apparatus comprising:
a rotatable mass that stores kinetic energy; a brushless generator having a pair output leads, said leads being coupled to said output, and at least one field coil, said generator being coupled to said rotatable mass such that kinetic energy from said mass is converted to electrical energy and delivered to said leads when field power is applied to said field coil; and a field controller that provides field power at a level to said field coil when said rotatable mass is rotating at normal full operating speed such that said generator provides full output power under full load to said output at the instant of disruption in primary power, and said field controller increases said field power during a continuous disruption of primary power to maintain a substantially constant output voltage at said output as said generator speed decreases.
1. An inductor alternator flywheel system comprising:
a rotatable member mounted on bearings for rotation about an axis, said member including a flywheel rotor with a hollow center and an inwardly facing radial surface forming a plurality of protrusions extending radially inwardly, said radial protrusions being circumferentially spaced apart with spaces between said protrusions free of magnetic material; a non-rotating member having a source of homopolar flux, said flux creating magnetic poles in said protrusions; at least one cylinder having an outer radial surface, said cylinder being mounted concentric with respect to said rotor and spaced apart radially from said protrusions such that an armature air gap is formed between said protrusions and said outer surface, said cylinder being constructed of substantially high permeability material; and at least one armature coil mounted in an air-core armature within said air gap such that said flux induces an alternating voltage in said armature coil when said rotor rotates about said axis.
37. An apparatus for providing emergency power in the event of disruption in power from a primary power source coupled to an output, said apparatus comprising:
a mass supported on bearings for rotation to store kinetic energy; a brushless motor/generator having a pair output leads, said leads being coupled to said output, and at least one field coil, said motor/generator being coupled to said rotatable mass such that kinetic energy from said mass is converted to electrical energy and delivered to said leads when said field coil is energized; and said brushless motor/generator also having multiphase armature coils from which said motor/generator is accelerated to normal operating speed by supply of AC power to said multiphase armature coils and is kept from accelerating above normal operating speed by reducing said AC power to said armature coils; said brushless motor/generator comprising laminated rotor and stator constructions providing high enough efficiency such that said field coil can be sufficiently energized such that the voltage generated by said armature coils remains substantially constant when primary power is disrupted; a field control mechanism increases power to said field coil to maintain a substantially constant voltage as the speed of said generator decreases.
17. An inductor alternator flywheel system, comprising:
a rotatable member mounted on bearings for rotation about an axis, said member including a flywheel rotor constructed of steel with a hollow center and an inwardly facing radial surface forming a plurality of protrusions extending radially inwardly, said radial protrusions being circumferentially spaced apart; said flywheel rotor has a central hole having a bore surface to which a hub portion is connected for mounting said flywheel to a central portion containing a bearing for journaling said flywheel rotor; said hub portion is constructed such that it provides a high reluctance magnetic path between said flywheel rotor and said center portion; a non-rotating member having a source of homopolar flux, said flux creating magnetic poles in said protrusions; at least one cylinder having an outer radial surface, said cylinder being mounted concentric with respect to said rotor and spaced apart radially from said protrusions such that an armature air gap is formed between said protrusions and said outer surface, said cylinder being constructed of substantially high permeability material; and at least one armature coil mounted within said air gap such that said flux induces an alternating voltage in said armature coil when said rotor rotates about said axis.
21. An apparatus for providing emergency power in the event of disruption in power from a primary power source coupled to an output, said apparatus comprising:
a rotatable mass that stores kinetic energy; a brushless motor/generator having a pair output leads, said leads being coupled to said output, and at least one field coil, said motor/generator being coupled to said rotatable mass such that kinetic energy from said mass is converted to electrical energy and delivered to said leads when said field coil is energized; and said brushless motor/generator also having multiphase armature coils from which said motor/generator is accelerated to normal operating speed by supply of AC power to said multiphase armature coils and is kept from accelerating above normal operating speed by reducing said AC power to said armature coils; a field control mechanism increases power to said field coil to maintain a substantially constant voltage as the speed of said generator decreases; a field controller that provides sufficient power to said field coil such that said output voltage does not drop appreciably during an interruption of said primary power without reduction in rotational speed of said motor/generator, and said field control mechanism further increases power to said field coil to maintain a substantially constant output voltage at said output as said generator speed decreases.
2. An inductor alternator flywheel system as described in
said flywheel rotor is constructed of steel.
3. An inductor alternator flywheel system as described in
said flywheel rotor is a hollow cylinder.
4. An inductor alternator flywheel system as described in
said inwardly extending protrusions are formed in a liner that is radially inside said hollow portion of said flywheel rotor.
5. An inductor alternator flywheel system as described in
said liner is interference assembled inside said hollow portion of said flywheel rotor.
6. An inductor alternator flywheel system as described in
said liner is constructed from multiple axial layers that reduce eddy currents in said protrusions.
7. An inductor alternator flywheel system as described in
said liner is comprised of laminations.
8. An inductor alternator flywheel system as described in
said liner has a lower maximum hoop direction stress than said flywheel rotor hollow portion when said flywheel rotor is rotating at normal operating speed.
9. An inductor alternator flywheel system as described in
said flywheel is connected to at least one hub having a smaller inner diameter, said inductor alternator flywheel further including a pair of upper and lower mechanical bearings for providing rotational support of said rotor, and an electromagnet mounted with respect to said flywheel rotor such that said electromagnet, when energized, lifts a majority of the weight of said flywheel rotor from said mechanical bearings.
10. An inductor alternator flywheel system as described in
said electromagnet acts directly on said flywheel rotor to remove a majority of the weight of said flywheel rotor from said mechanical bearings.
11. An inductor alternator flywheel system as described in
said source of homopolar flux comprises at least one field coil that induces said flux to flow.
12. An inductor alternator flywheel system as described in
a stationary yoke made of material having low magnetic reluctance, said yoke having an outer radial portion, an inner radial portion, and an intermediate portion between said outer radial portion and said inner radial portion; said intermediate portion of said yoke having surfaces that are closely coupled magnetically with said field coil; said outer radial portion of said yoke having a pole with axially facing surfaces closely spaced from said flywheel; said inner radial portion of said yoke having surfaces that are operatively closely linked magnetically with said cylinder; whereby said yoke, said flywheel, and said cylinder form a low reluctance magnetic circuit with said airgap between said flywheel and said cylinder, and said field coil drives flux through said magnetic circuit.
13. An inductor alternator flywheel system as described in
said cylinder is constructed of multiple axial layers that reduce eddy currents in outer surface of said cylinder.
14. An inductor alternator flywheel system as described in
said cylinder is comprised of laminations.
15. An inductor alternator flywheel system as described in
said laminations have a central hole and the inner diameter of said laminations is connected to a central shaft.
16. An inductor alternator flywheel system as described in
said central shaft is constructed of substantially high permeability material.
18. An inductor alternator flywheel system as described in
said hub portion is constructed from a non-ferromagnetic material.
20. An apparatus as described in
said generator is an inductor alternator with both rotor and stator comprising laminated constructions.
22. An apparatus as described in
the rotor portion of said motor/generator comprises multiple axial layer construction to reduce internal eddy current generation.
24. An apparatus as described in
said hollow cylinder is comprised of an alloy steel having been quenched and tempered to a tensile yield strength greater than 100 ksi and a fracture toughness greater than 70 ksi(in){circumflex over ( )}½.
25. An apparatus as described in
said hollow cylinder is constructed as a seamless ring by rolled ring forging.
26. An apparatus as described in
said liner is constructed from multiple pieces around its circumference.
28. An inductor alternator flywheel system as described in
said non-rotating member generating homopolar flux contains at least two field coils that generate flux across an upper and a lower axial air gap to said flywheel rotor; a position sensor determines the axial position of said rotor with respect to said non-rotating member; power to said field coils are actively controlled using information from said position sensor to form said axial magnetic bearing.
29. An inductor alternator as described in
increasing power to both coils increases the voltage induced in said armature coils, and increasing power to one field coil with respect to the other moves the flywheel vertically in one direction.
32. An inductor alternator flywheel system as described in
said cylinder is attached to said rotor through the use of a high reluctance connection.
33. An inductor alternator flywheel system as described in
said intrusions are constructed from multiple axial layers that minimize eddy currents in said intrusions.
35. An inductor alternator flywheel system as described in
a position sensor for determining the axial position of said rotor to provide axial control; and a magnetic circuit links said homopolar flux from said field coil and provides a bias flux for the axial magnetic bearing.
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This invention pertains to flywheel energy storage systems and more particularly to an improved inductor alternator flywheel system that eliminates power disruptions, has increased efficiency, and is more compact than previous devices. The higher efficiency of the invention allows its proficient operation with an activated field power, thus eliminating short-term interruptions of power occurring with prior systems and also potentially for allowing higher speed and higher power operation.
Flywheel energy storage systems are being employed for supplying power to a protected load during interruptions of power from the grid. Flywheel systems use a large inertia flywheel that is supported by a bearing system and is coupled to a motor/generator. The motor/generator converts between electrical and mechanical energy by accelerating the flywheel for storing energy and decelerating the flywheel for retrieving energy. The flywheel is typically housed in a low-pressure container to limit losses from aerodynamic drag. Such systems are very advantageous for supplying back-up power to communications networks during power outages, and can also be used as ride-through devices that supply power during the start-up period that an auxiliary power source, such as a generator, needs before it can come on-line. Flywheels offer very high power capability and increased reliability over conventional electrochemical batteries, and also have longer lifespans.
To date, various types of flywheel systems have been proposed, each having different goals and attributes for their different applications. One type of flywheel system that is currently being utilized employs an inductor alternator for the motor/generator. In general, alternators use field coils that create a magnetic field which can be varied by controlling power to the field coil, and thereby control the output voltage that is generated from rotation of an armature in the magnetic field. This control can be used to maintain a relatively constant desired output voltage for a flywheel system as the speed of the flywheel slows. In contrast, conventional permanent magnet generators suffer from an output voltage that decreases linearly with speed, necessitating the use of output power conversion electronics to maintain the desired constant output voltage. For very high power systems, the cost of power electronics increases substantially, thus making an alternator flywheel system desirable.
Inductor alternators are a type of alternator particularly well suited for flywheel and other high speed applications since they have both the field and armature coils stationary. This eliminates the need to reinforce coils for high-speed rotation and also the need for electrical contact with the rotating flywheel that would require brushes that wear. Inductor alternators generate power by inducing a voltage in the armature windings from periodic changes in the magnetic flux that links the armature coils. Current in the field windings sets up the flux, and changes in the flux through the armature windings are the result of changes in the reluctance of the magnetic circuit. The rotor of the inductor alternator causes these changes by rotation of teeth or poles in an air gap in the magnetic circuit, thereby increasing and decreasing the air gap as the teeth or poles pass through it. The rotor of an inductor alternator can thus be made very simply as a gear and as such is well suited for high-speed rotation When the rotor of an inductor alternator is combined into a flywheel, the resulting device can be well suited for both storing energy and for delivering a high power constant output voltage.
One such inductor alternator flywheel system of the prior art is shown in FIG. 1. The flywheel system 30 is comprised of a solid steel flywheel rotor 31 that rotates inside a shell 40. The flywheel 31 has integral shafts 36 that journal the rotor inside upper and lower mechanical bearings 37 and 38. The flywheel rotor 31 stores the kinetic energy and also serves as an inductor alternator rotor by having an outer surface defined by poles 32, each having a radius 34, separated by recesses 33 with radii 35. A stationary field coil 39 generates flux, which flows axially into the flywheel 31, then radially outward through the poles 32 and through armature coils 43. A laminated ring 42 diffuses changes in the flux created from rotation of the flywheel poles 32. The outer shell 40 that contains the vacuum also serves as a path for the flux to connect back to the field coil 39 and complete the magnetic circuit.
Although this type of inductor alternator flywheel system is relatively simple and is capable of supplying high levels of power, this system has several significant drawbacks. The system has power losses that generate heat during standby operation while rotating at full speed with no loss of primary power. To alleviate these losses, the power to the field coil is kept reduced when in standby mode. This reduces the flux through the rotor and its poles and the armature coils. In standby operation and also when the flywheel is accelerating, the armature coils are energized by synchronized multiphase power (typically 3 phase), to make the inductor alternator function as a motor. Energizing of a set of coils causes the rotor to rotate such that its poles or teeth tend to line up with the energized coils and when they do, the sequentially next coils are energized to propel it around. In the process, the energizing of armature coils and the rotation of rotor poles into a changing magnetic field, the magnetic flux through the rotor poles continuously changes. Changes in flux through any conductor, such as the steel rotor, cause generation of eddy currents. The large teeth allow a very big area for generation of large eddy currents. The greater the currents, the greater the loss and heat created. Losses also occur from the changing flux in the rotor due to magnetic hysteresis. Losses for the system shown in
In addition to these high losses and low storage issues, another problem with the system shown in
One well-known method for reducing losses in motor/generators and other magnetic circuit devices is to fabricate the magnetic path with laminated construction. The laminated construction builds the structure by stacking multiple layers of the magnetic circuit material together, each layer being electrically insulated from adjacent layers through use of insulative coatings. This construction reduces eddy current losses because eddy currents generated are limited in size to flow only in the thickness on the individual lamination layers. Loss reduction in motor/generators is also helped by specially selection the material of the magnetic path and its heat treatment. The material and condition are usually made to increase the magnetic permeability of the material, decrease its electrical conductivity and to reduce hysteresis losses. These steels typically have very low carbon contents. Different types of lamination or electrical steels can be selected based on the application, acceptable losses and acceptable costs. Silicon steels are very commonly used because the added silicon decreases the volume resistivity for further reduction of eddy current losses. Nickel alloys containing between 49% to 80% nickel with the remainder being nearly pure iron are used because of their very high permeability at low inductions and very low core losses. However, these alloys are very expensive. All types of lamination steels can be used to reduce the losses in inductor alternators. Unfortunately, lamination steels and their preferred heat treat condition for achieving high efficiency are not optimal for high speed operation with high stress levels. The strength of steels is directly related to its hardness condition. The maximum hardness that can be achieved with a given steel is directly related to the amount of carbon in the steel. These steels purposely have very low to almost no carbon in them. Further reducing the strength of these materials is the choice of heat treat condition for low losses. Lamination steels are used in the annealed condition. Annealing of steels results in the lowest possible strength. This is the opposite of quenching and tempering heat treatment preferably used to develop high strength for a high speed, highly stressed flywheel.
A high speed reluctance generator with a laminated rotor and stator of prior art is shown in FIG. 3. The reluctance motor/generator 50 is comprised of a rotor 53 that is fabricated from multiple axially stacked laminations 55, attached to a central shaft 56. A laminated stator 52, which also has poles 54, surrounds the rotor 53. The radial distance between the rotor 53 and stator 52 defines an air gap 57 of the machine. This use of the laminations allows for operation to very high rotational speeds, contemplated up to 100,000 rpm. The laminations reduce the losses such that a high frequency magnetic field from such high rotational speeds can be achieved. Although, the laminations in this rotor allow high rotational speeds, the rotor is not known to be capable of achieving high peripheral speeds. The diameter of the rotor would be limited by its ability to carry stress. A rotor of this construction having a large diameter and being rotated to high speeds would also pose difficulties in maintaining connectivity between the laminations and the central shaft. The stresses in a motor/generator rotor are proportional to the square of the peripheral speed, so if the diameter can be made small, the stresses can be kept low. For an energy storage flywheel, the goals are opposite: the peripheral speed is preferably made large for effectively storing a large amount of energy with the rotor.
The invention provides an inductor alternator flywheel system that has increased efficiency and is more compact than previous devices. The higher efficiency enables the system to operate with fully activated field power, allowing it to supply power without interuption in the event of a power failure in the grid. The higher efficiency also provides the potential for the system to operate at higher speed and higher power. The inductor alternator flywheel system uses a hollow steel cylindrical flywheel with the inductor alternator in the center. The bore of the hollow cylinder contains inwardly projecting teeth or castellations, which serve as the poles of the inductor alternator. A central cylinder, preferably having at least a partially laminated construction, cooperates with the rotor bore to form the stator. Armature coils are located radially between the central cylinder and rotor bore. At least one separate field coil provides the flux for alternator operation. The construction allows current to this field coil to be kept fully energized when the flywheel is rotating, even in standby operation. The flywheel is accelerated to full speed and kept at that speed by regulating the power from a synchronous inverter that drives the armature coils. During an interruption of primary power, the flywheel system instantly supplies full load power to the output without the time delay that occurs in prior art systems while the low stand-by current in a field coil is increased to establish the fill field flux necessary to generate fill power. As the speed is reduced from energy being extracted, the power to the field coil is further increased to maintain a constant output voltage until most of the energy is extracted.
Building a steel flywheel with a central hole causes the hoop direction stress to increase by a factor of two or greater. Metal flywheels that are constructed to be solid, without a central hole, have the lowest internal stress and can be operated at the highest rotational speed. Nevertheless, use of a hollow center in this invention is preferable, as will be shown. Prior inductor alternator flywheel systems are operated at relatively low stress levels; it is the system efficiency, and not the flywheel integrity, that is the main factor limiting performance. Placing the inductor alternator inside the bore allows a larger majority of the flywheel mass to be located at a large diameter, thereby maximizing the inertia per size and weight of the flywheel. With the inductor alternator integrated in the center, the system is also made more compact.
Further adding to the benefits of the invention, the construction provides the potential for maximizing efficiency. Because the flywheel is made as a hollow cylinder, the inductor alternator and flywheel energy storage functions can be partially separated such that their performances can be improved independently. The inductor alternator rotor with inwardly projecting poles can be made as a liner that is inserted into the bore of the flywheel rim, so the flywheel rim can be made from a high strength steel and heat treated to a high hardness while the liner is made from a lower magnetic loss material that has lower strength. The flywheel rim can be used to reinforce the liner for high speed rotation and the liner allows the inductor alternator to efficiently operate at high speeds. The liner can be interference fit into the bore of the flywheel rim by shrink fitting or press fitting which drives the liner initially into compression. During high-speed rotation, the stresses in the liner are thus kept low and acceptable for the liner material. Although the flywheel rim outside the liner may be used as part of the magnetic circuit, the flux through the rim is nearly uniform after passing through the liner and hence does not induce significant eddy currents that would be induced by a fluctuating magnetic flux, so losses are minimized.
In one embodiment of the invention, the liner is contrasted from multiple axially stacked layers that reduce the eddy currents in the pole teeth. The layers of the liner can be constructed of steel and separated by layers of electrically insulating material. For the lowest magnetic losses, the axial layers of the liner are constructed of very thin layers, preferably laminated steel. The thin laminations are electrically insulated by oxidizing or coating with enamel type coatings prior to stacking. The thinner the laminations for a given liner material, the lower the losses in the flywheel system but a balance must be made based on the costs.
In yet another embodiment of the invention, the hoop stress in the liner is further reduced to near zero stress by fabrication the liner from multiple individual segments about its circumference. This precludes the development of hoop stresses in the liner and can also reduce the cost of the liner. Instead of having to stamp rings where the center material is wasted, better material usage can be achieved by stamping the liner elements as arc segments.
The flywheel/inductor alternator rotor can be supported for rotation in the flywheel system by several different methods. One method involves connecting smaller diameter bearings in a hub that is mounted in the bore of the flywheel rim, and supporting those bearings on a central fixed shaft. To prevent shorting out the flux from the field coil through the hubs, they are preferably constructed from a non-ferromagnetic material such as aluminum or they contain a section of high reluctance for the field flux. The hubs can be interference assembled with the rim, or use sliding or bending joints to maintain connection with the rim at high speed. The inductor alternator flywheel system can employ upper and lower field coils or alternatively only one field coil can be used. In one version of the invention, the weight of the flywheel rotor is born by permanent magnets attached to the rotor that are in repulsion with stationary permanent magnets. Another version uses a central hub, which divides the rotor in two sections along its axial length. Upper and lower armature coils are employed, creating an upper and lower inductor alternator. In another version of the invention, the center cylinder is attached to the flywheel and rotates with it, also serving as the shaft for journaling between upper and lower bearings. The shaft is again attached to the flywheel rim using a high reluctance hub.
In one configuration of the invention the flywheel rotor is supported using full levitation magnetic bearings. The flywheel is supported radially by upper and lower radial magnetic bearings. A separate axial magnetic bearing can be used lift the flywheel weight. However in one embodiment of the invention, and this embodiment could be used with other types of inductor alternator flywheel systems, the field coil or coils are used to provide the active axial magnetic bearing in a fully levitated system. Although prior art systems used the field coils to reduce the weight of the flywheel that is carried by mechanical bearings, these systems used load force to provide feedback or were set as a constant lift force by supplying a constant current. For a fully levitated flywheel rotor, a constant current does not create a dynamically stable system. An axial position sensor is preferably used to sense the rotors position and to provide feedback for axial levitation. Because the field coil in an inductor alternator system is preferably wound to be very large and have a large inductance so that a high field can be created with low power consumption, the response time of the coil is very slow. The response time was not important for systems that only removed a portion of the load because the rotor position was essentially fixed and not dynamically unstable. However, for a fully levitated rotor, controlling the current to the field coils to achieve axial levitation would be extremely difficult if not impossible. The field coils do set up large axial direction fluxes that generate enough force to levitate the flywheel. The invention allows for these field coils to provide levitation force by replacing a single field coil with two coils. One coil is very large and it provides the flux for the inductor alternator operation. This flux is used as a bias flux for the magnetic bearing. The second coil is smaller, with a much lower inductance and faster response time, and it is used to provide the axial position control using feedback from the axial position sensor. The control force from the smaller coil is amplified by having the bias flux since the force is proportional to the square of the flux density.
The invention and its many attendant benefits and advantages will become better understood upon reading the following description of the preferred embodiments in conjunction with the following drawings, wherein:
As
Turning to the drawings, wherein like characters designate identical or corresponding parts, an inductor alternator flywheel system 60 in accordance with the invention is shown in FIG. 4 in a basic form without bearings and other auxiliary components to clearly illustrate the functioning of the inductor alternator flywheel. The system uses a steel flywheel having a flywheel rim 61 in the form of an annular cylinder. An inductor alternator 62 for accelerating and decelerating the flywheel is integrated into the bore of the annular flywheel rim 61. The flywheel rim 61 has intrusions or inwardly extending radial projections 63 that act as the poles for the rotor of the inductor alternator 62. The inductor alternator stator is comprised of several pieces which include a laminated center cylinder 64, solid central shaft 65 and upper and lower yokes 66 and 68.
Upper and lower field coils 70 and 71 are used to drive magnetic flux through the yokes 66, 68, center shaft 65, radially outward through the laminated cylinder 64, through stationary armature coils 72, across a radial airgap 73, into the rotor 61 and axially outward through the axial air gaps 74 and 75. The flux crosses the axial airgaps 74, 75 and returns to the stator yokes 66, 68 though the axial poles 67 and 69. As the rotor rotates, the flux through the armature coils 72 changes due to the effects of the rotor poles or intrusions 63. The armature coils can be slot wound however, they are preferably air core such that they have reduced inductances for faster current rise times.
The inductor alternator flywheel system operates as follows. The flywheel is accelerated by applying synchronous AC power to the armature coils. The flywheel is kept rotating at its normal operating speed when primary power is functioning and power is continuously applied to the field coils. When primary power is interrupted, the inductor alternator supplies power to prevent a power interruption. As the speed of the flywheel decreases from energy being extracted, power to the field coil is increased to maintain a constant output voltage until most of the energy is used.
Normally the design of high speed flywheels that are constructed from metal such as steel are purposely made to be solid without a central hole. Adding a hole to a rotating metal disk causes the hoop stresses to more than double, limiting energy storage capability. Contrary to this well accepted convention, the invention achieves significant benefits from having a hollow center. Inductor alternators of prior art operate at relatively low tip speeds and stress levels. They also have high energy losses even at these speeds and power to the field coil or coils is reduced when in standby operation so as to reduce the losses. Increasing the operating speed of these systems would increase losses and eddy current losses in the rotor poles with the square of the speed. Locating the stator of the inductor alternator outside of the rotor is useful for easily removing the generated heat, however this makes the system more bulky. Considering the limited operating speed and the fact that the portion of the rotor in the center stores little energy due to its low inertia, the invention instead places the inductor alternator in the center of a hollow cylinder flywheel to create a much more compact system. Besides being smaller, the energy storage per weight is increased due to having a larger portion of the flywheel mass located at a larger diameter.
The top view of an inductor alternator flywheel of
As mentioned previously, having a central hole in the flywheel causes its maximum hoop stress to be more than double that of a solid disk with the same outer diameter and rotational speed. The radial and hoop direction stresses for an inductor alternator flywheel in accordance with the invention are shown in
The invention allows for a more compact inductor alternator flywheel system with better energy storage per weight. Despite having higher stresses in the flywheel than previous designs, the flywheel rim is physically capable to rotate to even higher speeds. To proficiently operate at higher speeds, a second embodiment of the invention allows for greatly increasing the efficiency as will be shown. Because the inductor alternator is located at the center of the flywheel, the inductor alternator and flywheel rim can be separately optimized. The poles of the inductor alternator rotor can be formed in a liner and this liner can be attached to the inner diameter of the flywheel rim. This allows for the liner to be made from a low loss material and construction. The rim, which surrounds the liner and reinforces it, can thus be simultaneously made from a high strength steel and heat treated for maximum energy storage. The low magnetic loss liner complements the higher speed capability of the rim by having lower losses that allow for higher speed rotation. More importantly than increasing the operating speed, the lower losses from the inductor alternator allows the system to be acceptably operated with a fully energized field coil in normal operation. The field coil power is kept on such that when primary power fails, the inductor alternator instantly and automatically supplies fill power to the load. No time delay occurs from having to ramp up power to the field coils. Although systems of prior art could physically be run with full field coil power or alternatively using permanent magnets for a continuous field, the losses would be very high. The disclosed preferred method for operation of those systems is to keep the field coil power at a reduced value until the primary power fails and is detected. Obviously, this method of operation could also be used with the invention but it would have the drawback of having an interruption of power, which is sought to be eliminated. As the flywheel slows from energy being removed, the power to the field coil is further increased to maintain a substantially constant output voltage. If a constant output voltage were not required, the field coil power could be left constant.
The liner can be constructed numerous ways. The liner can achieve lower losses simply by using a lower loss material such as annealed silicon steel or even a ferrite. The liner would preferably be machined such that it would have the intruding rotor poles and then would be installed in the rim. Depending on the elastic modulus, density and strength of the liner, it can be bonded in place or alternatively it may require interference assembly into the rim bore. Shrink fitting of press fitting can do interference assembly. The top view of an inductor alternator flywheel with internal liner in accordance with the invention is shown in FIG. 9. The steel flywheel rim 101 has an internal liner 103 that contains the rotor poles 104. A stator comprised of an inner cylinder 105, preferably of laminated construction, and a central steel shaft 106 are located inside the liner 103. Air core armature coils 116 are attached to the outer diameter of the stator cylinder 105 and a radial air gap is formed between the outer diameter of the armature coils 116 and the inner diameter of the rotor poles 104. The corners 117 of the poles 104 can be left sharp or they can be rounded so as to reduce generated harmonics.
The radial and hoop stresses in a flywheel rim and liner using a shrink fit in accordance with the invention are shown in
In another embodiment of the invention, the rotor liner is constructed from multiple axially stacked layers, thereby enhancing the efficiency of the inductor alternator. As shown in
The alternator 82 includes a fixed stator having a laminated center cylinder 85 attached to a center steel shaft 86. A plurality of armature coils 94, each wound on an axis oriented radially and facing the rotor poles 84, are located in the air gap 95 between the outer diameter of the stator cylinder 85 and the inner diameter of the rotor poles 84. Upper and lower field coils 92 and 93 produce magnetic flux, and upper and lower yokes 88 and 90 provide a flux return path. The flux through the rim and the yokes and center shaft is maintained essentially uniform, homopolar, and does not change with the rotation of the rotor. Because of this, these structures need not be constructed of low loss magnetic materials to reduce losses. The laminated stator cylinder 85 and low loss rotor liner 83 diffuse varying fluxes before the flux enters the center shaft 86, rim 85 or yokes 88 and 90. The flux exiting the rim exits through upper and lower axial air gaps 96 and 97 and enters yoke poles 89 and 91. Although it is conceivable that the yoke poles 89,91 could be made to act directly on the axial ends of the liner 83, this could result in a larger effective air gap due to the insulating layers 87.
The concept of constructing the liner 83 from multiple axially stacked layers for reduction of losses can be taken a step further, as illustrated in
The stator includes a laminated center cylinder 105 and a center steel shaft 106, and is located concentric to the flywheel 101 and in its hollow bore. Armature coils 116 attached to the outer diameter of the stator cylinder 105, with their axes oriented radially, provide for power conversion. As shown in
Although the hoop direction stresses in the rotor liner can be made very small through the use of an interference fit with the rim, an alternate configuration and embodiment of the invention can reduce these stresses essentially to zero. Instead of making the liner as a single ring construction, whether it is solid or an assembly of axial layers, the liner can be made from multiple segments around the circumference. A top view of an inductor alternator flywheel with multiple circumferential piece liner construction in accordance with the invention is shown in FIG. 11. The flywheel rim 120 has an internal liner constructed from multiple pieces 121. Inwardly extending protrusions or teeth form the rotor poles 122 for operation as an inductor alternator. As the flywheel 120 is rotated to high speed, the liner pieces cannot develop significant hoop direction stresses because they are not from a continuous piece. The liner pieces 121 can be bonded in place or alternatively mechanically fastened such as by use of an axial clamping mechanism, not shown. Besides reducing hoop stresses in the liner, the use of multiple piece construction has the potential for reduced costs. Making the liner from a single ring construction results in wasted material from the center. Making the liner from smaller arc segments 121 can provide better material utilization. As before, the stator with laminated cylinder 125, center shaft 126 and armature coils 124 are located in the center of the rotor. The radial air gap 123 is formed between the inner diameter of the liner and the outer diameter of the stator cylinder 125.
The invention has been shown as a flywheel and integrated induction alternator so far. Now, a brief description is provided of some illustrative examples of incorporating the flywheel and integrated induction alternator in a complete flywheel power system. These examples are not intended to be exhaustive and other structures will occur to those skilled in the art in light of this disclosure, and these other structures are intended to be encompassed by the spirit and scope of the claims. Two components useful for operation of a flywheel electrical power system include bearings for supporting the flywheel for rotation, and a container for maintaining a vacuum around the flywheel.
One complete inductor alternator flywheel system 130 in accordance with the invention, shown in
The rotor liner 135 has laminated construction and is interference assembled into the bore of the rim 131. Teeth 136 in the liner 135 create the rotor poles in the manner shown in
An alternate configuration of an inductor alternator flywheel system 160 in accordance with the invention, shown in
A laminated rotor liner 164 with internal poles 165, as described in
Another configuration of the invention can be achieved with the inductor alternator divided into two individual units, as shown incorporated in a flywheel system 190 in FIG. 14. Dividing the system into two inductor alternators can have the advantages of allowing one to function as the motor and one as the generator, mechanically balancing the system, or for providing multiple different output voltages. The flywheel system 190 includes a hollow steel flywheel 191 that rotates inside a container 192 having an internal chamber 193 maintained at low pressure. The flywheel rim 191 is connected to a central shaft 195 by way of a center non-ferromagnetic hub 194. The central shaft 195 is journalled for rotation in upper and lower bearings 206 and 216. The bearings 206, 216 are attached with spring dampers 207 and 217 to upper and lower bearing mounts 205 and 215. The bearing mounts 205, 215 also support the upper and lower stator laminations 204 and 214 and provide axial flux paths to upper and lower stator yokes 211. Upper and lower inductor alternators 197 and 196 are formed at the respective ends of the flywheel 191. The stators 204, 214 contain armature coils on the outer diameter and radial air gaps 198 and 199 are formed between them and the inner diameter of the rotor liners 200 and 202. The rotor liners 200, 202 contain inner poles 201 and 203, shaped as shown in
Longer life and or higher speed operation can be achieved with the inductor alternator flywheel system by using full levitation magnetic bearings. An inductor alternator flywheel system 230 using such a full levitation magnetic bearings in accordance with the invention, as shown in
The flywheel 231 is supported for rotation using a fill levitation magnetic bearing system having upper and lower magnetic bearing rotors 246 and 244 that cooperate with upper and lower magnetic bearing stators 247 and 243. The magnetic bearing stators 243, 247 are attached to the center shaft 239. The magnetic bearing rotors 244, 246 are attached to upper and lower hubs 245 and 242. The hubs 242, 245 have an angled configuration that allows them to bend and stay connected to the inner diameter of the rim 231 despite radial growth of the rim at high speed. Also attached to the inner diameter of the hubs 242, 245 are mechanical auxiliary bearings 243 and 248 that provide support when the magnetic bearing system is not operating. The upper and lower magnetic bearings 246, 247, 244, 243 provide radial support for the flywheel 231. At least one axial magnetic bearing must be used to provide fully stable five axes levitation. The axial magnetic bearing can be added separately or could be combined with the radial magnetic bearings 246, 247, 244, 243.
Alternatively, the inductor alternator field coils 259 and 262 can provide the function of the axial magnetic bearing. This embodiment could also be applied to other designs of inductor alternator flywheel systems. The field coils 259, 262 provide flux through yoke poles 256 and 260 forming axial air gaps 257 and 261. The field coils support the entire weight of the flywheel 231, providing full axial levitation Removing load from mechanical bearings could be done by simply using a load cell to measure the bearing force or using a constant current difference between the upper and lower field coils. However, for full levitation the flywheel becomes axially free and achieves dynamic stability and stable levitation. An axial position sensor, not shown, is preferably used to provide control feedback. The feedback could directly control power to the field coils 259, 262. Unfortunately, the feed coils 259, 262 are preferably wound to be large, having many turns and a high inductance, such that they can generate a substantial flux for operation of the inductor alternator with low field power consumption. The large inductance of the field coils 259, 262 gives them a very slow response time. This makes control for full levitation extremely difficult. To solve this problem, the flux from the field coils 259, 262 is used to provide bias flux for the axial magnetic bearing. Separate smaller control coils, which can be located adjacent the field coils 259, 262, are provided having lower inductances and faster response times. These control coils provide axial control that is amplified from having bias flux of the field coils 259, 262 because the forces generated are proportional to the square of the flux densities. The control coils could also be located in other positions to add flux to the flux in the air gaps 257 and 261, still using the field coils 259, 262 for providing bias flux.
In all of the flywheel system configurations thus far, the inductor alternator flux has traveled radially outward through a stationary portion. It is also possible to make the center cylinder, from which flux exits to pass through the armature coils, rotating with the flywheel. Such an inductor alternator flywheel system 270 with a rotating center portion in accordance with this aspect of the invention is shown in FIG. 16. The flywheel system 270 is comprised of a steel flywheel rim 271 that rotates inside an evacuated chamber 273 within a sealed container 272. In this configuration, the flywheel 271 has an upper 280 and a lower 281 inductor alternator on opposite sides of a low permeability hub 275. As shown, the upper and lower inductor alternators have different constructions to illustrate different possibilities, although in an actual installation both upper and lower inductor alternators would normally be the same construction.
The rim 271 is attached to a central shaft 274 by way of the hub 275. The shaft 274 has upper and lower bearing stub shafts 277 and 276. Mechanical bearings 278 and 279, which are mounted in the upper and lower yokes 290 and 289, connect with the bearing shafts 277, 276 to support the flywheel 271. The upper inductor alternator 280 shows a very simple construction, but one having higher losses. The flux from the upper field coil 292 travels through the inner part of the upper yoke 290 and enters the upper axial end of the central shaft 274. It passes through the shaft 274 and exits radially outward from the outer diameter of the shaft 274. The flux travels through an inner airgap 282, through the armature coils 286, through a second air gap 283 and into the bore of the rim 271. The inside of the rim 271 contains intruding poles 284. The flux then travels through the rim 271 and across an upper axial air gap 293 into the stator yoke 290. Because the central cylinder or shaft 274 rotates with the rim 271, the flux intensity in those parts does not vary significantly with time, so losses are low. The annular flux pattern in the airgap containing the stationary armature coils 286 has alternating high intensity and low intensity bands coinciding with the poles 284 which rotate with the shaft 274 and the rim 271. The stationary armature coils 286 see large changes of flux intensity as the shaft and rim rotate, but the flux in the shaft 274 and rim 271 does change significantly with time, so the losses due to eddy currents are low.
The bottom inductor alternator uses a laminated construction to obtain the highest efficiency. A laminated annular center cylinder 295 is connected to the center shaft 274 and a laminated rotor liner 288 is attached to the bore of the rim 271. The armature coils 286 are in the air gap radially between the center cylinder 295 and the rotor liner 288 using supports 287. The laminated rotor liner 288 has internal poles 285. Flux is generated from the lower field coil 291 and travels axially into the end of the center shaft 274. The flux returns to the lower yoke 289 through the lower rotor axial air gap 294. The upper and lower yokes 290, 289 are preferably magnetically insulated from each other.
Another configuration of the invention can be constructed in which the flywheel is not hollow throughout its axial length and the field does not travel through an external yoke. An inductor alternator flywheel system illustrating these aspects of the invention is shown in FIG. 18. This configuration has the benefits of making a compact system and also minimizing manufacturing and assembly costs. The flywheel system 330 is comprised of a steel flywheel 331 that rotates inside an evacuated chamber 333 within a sealed container 332. The flywheel 331 has an integral center shaft 335, however it is possible to also construct the shaft and the center region of the flywheel separately and assemble them together. The shaft 335 is supported for rotation on upper and lower beatings 338 and 336. The bearings 338, 336 are supported using upper and lower bearing mounts 339 and 337. The top bearing mount 339 is preferably made including a low magnetic permeability material so it prevents shorting of the field flux in the container 332, if the container 332 is constructed from steel. The inductor alternator 340 has a stationary central stator cylinder 350 made with laminated construction. Because the center shaft 335 rotates, an inner radial air gap 342 is provided. Armature coils 345 are attached to the outer diameter of the stator cylinder 350. The flywheel 331 is provided with a laminated rotor liner 343, having internal poles 344, like those shown in
Another embodiment shown in
The inductor alternator flywheel system is used for preventing interruptions of power to an output load. It functions as an uninterruptible power supply with the flywheel system doing the functions normally done by electrochemical batteries. A circuit diagram of an interruptible power source in accordance with the invention is shown in FIG. 17. The uninterruptible power source 300 is connected to a primary power source 301, which is typically AC utility power. The power source 300 then supplies output power 312 to a critical load, not shown. In applications where protection is to be provider for against extended interruptions, an auxiliary power source 304, such as a generator, can be included. The auxiliary power source 304 is connected through use of a transfer switch 302 when the primary power source 310 fails for an extended period. The AC primary power 310 is rectified to a DC Buss 305 through use of a rectifier 303. The DC Buss power 305 is then inverted back to the AC output power 312 through use of an inverter 311. If the output power required were DC, the inverter 311 could be omitted. During an interruption of primary power 301 and prior to availability of auxiliary power 304 if included, the inductor alternator flywheel 307 provides power. When primary power is operative, the flywheel is accelerated to its normal operating speed. Acceleration and maintaining of speed is done achieved by using a synchronous inverter 306 that provides synchronous power to the armature coils 308 to provide acceleration. The inverter 306 preferably controls power to the armature coils 308 such as current limiting and control of the speed. One method to control power is through use of pulse width modulation. It is also conceivable that the power to the armature coils 308 could be applied as pulsed DC for providing acceleration since the rotor in the inductor alternator flywheel system is made to be homopolar. This would have the advantage of simplifying the synchronous motor drive 306. Because of the high efficiency of the invention, it is desirable to operate the inductor alternator 307 with fully energized field coils 309. During an interruption of primary power 301, the field is already fully established. Power is thus instantly and automatically supplied to the DC Buss and output power 312. Power immediately flows from the armature coils 308 through rectifier 310 to the DC Buss 305. No monitoring of the primary power 301 is required and no interruption in power from the inductor alternator 307 or in the output power 312 occurs. The rectifier 310 can be separate as shown or it can be combined in the inverter or motor drive 306. As the flywheel speed decreases, a field controller 313 increases power to the field coils 309. A preferred way to control the field power is from feedback using the flywheel speed 314. Other ways such as monitoring the output power 312 or DC Buss power 305 could alternatively be done. A majority of the flywheel energy is preferably extracted to support the output 312. During that period of discharge, either primary power 301 is restored or the auxiliary power source 304 is transferred on to supply power. The auxiliary power source 304 is preferably kept running until both the primary power source 310 is restored and the inductor alternator flywheel 307 is recharged. Although the invention could conceivably be operated with a reduced field coil power when primary power is functioning, this would create an interruption in power that is sought to be eliminated. The higher efficiency of the invention enables its operation preventing all interruptions of power and also to potentially operate with high speed and energy storage capability.
Obviously, numerous modifications and variations of the preferred embodiment described above are possible and will become apparent to those slilled in the art in light of this specification. For example, many functions and advantages are described for the preferred embodiment, but in some uses of the invention, not all of these functions and advantages would be needed. Therefore, I contemplate the use of the invention using fewer than the complete set of noted functions and advantages. Moreover, several species and embodiments of the invention are disclosed herein, but not all are specifically claimed, although all are covered by generic claim. Nevertheless, it is my intention that each and every one of these species and embodiments, and the equivalents thereof be encompassed and protected within the scope of the following claims, and no dedication to the public is intended by virtue of the lack of claims specific to any individual species. Accordingly, it is expressly intended that all these embodiments, species, modifications and variations, and the equivalents thereof are to be considered within the spirit and scope of the invention as defined in the following claim, wherein I claim:
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